Increased terrestrial ecosystem disturbances during the onset of the PETM and associated carbon cycle perturbations

The intensification of the hydrological cycle, driven by global warming is expected to amplify extreme weather events and associated erosion. These hydrological shifts are likely to disrupt soil stability, accelerate organic carbon mineralization, and alter terrestrial ecosystems, all of which have potential implications for carbon cycle dynamics. Such terrestrial carbon cycle feedback mechanisms remain poorly constrained. While relatively slow compared to present-day carbon cycle change, the millennial scale onset of the Paleocene-Eocene Thermal Maximum (PETM, ~56 million years ago) was similarly associated with a massive input of ¹³C-depleted carbon into the ocean-atmosphere system that is recorded by a negative carbon isotope excursion (CIE) in sedimentary components. The PETM marks a global temperature increase of ~5 °C and is characterized by significant associated hydrological disturbances, erosion, and vegetation changes; the precise timing, impact and spatial scale of these processes are still being debated. Here, we show vegetation shifts  at the Norwegian Margin during the CIE onset interval at centennial scale resolution. Furthermore, we show terrestrial disturbances during the CIE onset are synchronous along several continental margins globally based on organic microfossil assemblages and reworked soils (clay minerals and organic matter). These observations signal changes in terrestrial biomass, intensified oxidation of soils and weathering of fossil organic carbon, potentially acting as a positive carbon cycle feedback mechanism. Carbon cycle model simulations indicate that these shifts in terrestrial carbon storage and fluxes may have appreciably contributed to the CIE and climate change, highlighting the importance of constraining the response of terrestrial biosphere feedback mechanisms to changing weather and climate.